A neuropsychological profile of dementia was drafted for individuals with Alzheimer's disease (AD), Huntington's disease (HD) and "at risk" for Huntington's utilizing standard and experimental tasks, also establishing normative references for functional changes accompanying the aging processes. Although AD is accompanied by marked deficits in selective attention, memory and learning, there were no qualitative differences between demented and age-matched subjects. The impairment also extend to object-naming and fluency, and AD patients performed poorly in perceiving meaning, except when the stimuli required emotional judgement. The data indicate the Alzheimer's patients may be unable to encode material; this is in sharp contrast with other amnesic disorders where the primary difficulty involves an inability to store and/or retrieve information. AD and HD patients showed pronounced but dissimilar deficits with visuospatial and constructional tasks. The behavioral data extend neuropathologic impressions of involvement in AD. The neuropsychological test profile of AD patients yielded different clinical subgroups or populations. Memory and learning deficits, per se, were poor indicators of group membership. One group was characterized by severely impaired verbal abilities, but with intact perceptual and constructional skills. The second group was more impaired on perceptumotor than verbal tasks. The third group showed comparable deficiencies in both linguistic and visual spatial sectors. Positron emission tomographic and EEG data confirmed corresponding changes in left, right or bilateral regions in the posterior cerebral quadrant, respectively.